This invention relates to forming a conversion coating on an aluminum surface which replaces the native aluminum surface oxides. More particularly, the conversion coating is formed by first treating the aluminum by thermal degreasing or aqueous alkaline degreasing and then exposing the treated aluminum to an aqueous KF solution. This coating has various uses, e.g., as an aid for fluxless brazing, as a durable abrasion. resistant surface with good lubricity, and to modify surfaces for other purposes.
Workers in the field often use brazing to join aluminum alloy components and in particular, brazing is the preferred method for joining aluminum heat exchanger assemblies. Aluminum and aluminum alloys, however, are generally very reactive and rapidly form a surface oxide film (5-100 nanometers thick) when exposed to the atmosphere at ambient temperatures. Such oxide film inhibits forming a strong, high quality, defect-free bond between the articles to be joined. Thus, to effect a metallurgical, chemical or intermetallic bond between the aluminum or aluminum alloys, it is considered necessary to remove, dissolve or disrupt such oxide film. For example, the oxide layer may be removed by chemical etching using a KF etchant as discussed in U.S. Pat. No. 5,820,015. It teaches particularly that the fillet-forming capability of brazeable aluminum alloy can be improved by such etching. Another common way to disrupt the oxide layer is with a flux like NOCOLOK(trademark) applied to the joining surface. It is an aqueous suspension of KAlF4+K3AlF6 solution which can sprayed onto degreased aluminum parts, and after drying, the parts can be brazed in a furnace. The flux melts at 565-575xc2x0 C. and reacts with the surface aluminum oxide. This causes the surface to be both dissolved and simultaneously protected from further oxidation, as it reaches braze temperature between 585-600xc2x0 C. One disadvantage of this method of using a flux, however, is the inefficiency of applying the flux. That is, the flux powder needs to be suspended in a liquid vehicle, the surface sprayed before and after assembly, and then dried. The powder flux can also cause housekeeping problems at the manufacturing facility. To overcome problems associated with fluxes, fluxless brazing has been developed.
In one type of fluxless brazing, a flux layer is deposited by chemical conversion on the surface of the aluminum to be brazed. On such method is disclosed in EPO 0140 267 B1 where it is taught that the layer formed is composed of potassium pentafluoroaluminate (K2AlF5). In the ""267 method, the brazing step includes heating the flux layer below the melting point of the aluminum but higher than the melting point of a brazing alloy to join the aluminum to a counterpart material with the brazing alloy. Providing a completely leak free joint as necessary in applications like heat exchanges is difficult with this process.
We have now unexpectedly found that a flux-less brazeable aluminum or aluminum alloy surface can be prepared which overcomes the deficiencies of prior art processes. It particularly involves an aluminum or aluminum alloy surface which has been treated with thermally degreasing or an alkaline aqueous degreasing. This step was unexpectedly found, in combination with a conversion coating step using an aqueous KF bath, to be critical to providing sound braze joints.
While conversion coatings useful for fluxless brazing may be provided, the invention coating was unexpectedly found useful in other ways. For example it may be used to provide on the aluminum surface a durable, abrasion resistant coating with good lubricity characteristics. This coating is then advantageous on aluminum surfaces which during use are exposed to sliding friction, e.g., swashplates of air conditioning compressors. The conversion coating can also be used as a surface passivator prior to adhesive bonding of the aluminum surface. These and other aspects of the invention will be discussed in detail below.
The invention is a method for providing a conversion coating on an aluminum (meaning also aluminum alloy) surface. The method requires providing an aluminum surface which has been treated by thermal degreasing or aqueous alkaline degreasing, such that the treated surface is substantially devoid of grease and oils. It is critical to the method that one of these two particular treatments are used. Subsequently the treated surface is contacted with an aqueous bath solution of 2.0-25.0 wt. % KF at a temperature of 90xc2x0 F.-212xc2x0 F. for at least 5 seconds to provide a conversion coating on the aluminum surface consisting essentially of: (a) substantially all K3AlF6 or (b) a major proportion of K3AlF6 and a minor proportion of KAlF4 Preferably, to provide optimal fluxless brazing coatings, the KF concentration of the bath is about 2.5 to 5 wt. % KF. For providing an abrasion resistant coating, optimally the KF bath concentration is 3-10 wt. % KF.